Holographic storage systems are storage systems that use holographic storage media to store data. Holographic storage media includes photorefractive materials that can take advantage of the photorefractive effect described by David M. Pepper et al., in “The Photorefractive Effect,” Scientific American, October 1990 pages 62-74. The index of refraction in photorefractive materials can be changed by light that passes through them. Holographic storage media also include photopolymers, such as those described in Coufal et al., “Photopolymers for Digital Holographic Storage” in HOLOGRAPHIC DATA STORAGE, 199-207 (2000), and photochromic materials. By controllably changing the index of refraction in such materials, high-density, high-capacity, and high-speed storage of information in holographic storage media can be accomplished.
In the typical holographic storage system, two coherent light beams are directed onto a storage medium. The first coherent light beam is a signal beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a signal beam containing the information stored is produced. Most often the appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image. More than one holographic image may be stored in the same volume by, for example, varying the angle of the reference beam during recording.
Varying the angle of the reference beam during recording to store multiple holographic images in the same volume is called angle multiplexing. In addition to angle multiplexing, other techniques for storing multiple holograms in the same volume include wavelength multiplexing, phase code multiplexing, correlation multiplexing, shift multiplexing, aperture multiplexing, and fractal multiplexing. Since the same volume can be used to store multiple holographic recordings, high storage capacities can be obtained using holographic storage systems.
Information can be encoded within the signal beam in a variety of ways. One way of encoding information into a signal beam is by using an electronic mask, called a spatial-light modulator (SLM). Typically, a SLM is a two dimensional matrix of pixels. Each pixel in the matrix can be directed to transmit or reflect light, corresponding to a binary 1, or to block light, corresponding to a binary 0. The signal beam, once encoded by the SLM, is relayed onto the storage medium, where it intersects with a reference beam to form an interference pattern. The interference pattern records the information encoded in the signal beam to the holographic storage medium.
The information recorded in the holographic storage medium is read by illuminating the storage medium with a reference beam. The resulting signal beam is then typically imaged onto a sensor, such as a Charge Coupled Device (CCD) array or a CMOS active pixel sensor. The sensor is attached to a decoder, which is capable of decoding the data.
FIG. 1 illustrates the basic components of a holographic system 100. System 100 contains a SLM 112, a holographic storage medium 114, and a sensor 116. SLM 112 encodes beam 120 with an object image. The image is stored by interfering the encoded signal beam 120 with a reference beam 122 at a location on or within holographic storage medium 114. The interference creates an interference patterns (or hologram) that is captured within medium 114 as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 122, depending on the particular reference beam employed. Signal beam 120 typically passes through lenses 130 before being intersected with reference beam 122 in the medium 114. It is possible for reference beam 122 to pass through lenses 132 before this intersection. Once data is stored in medium 114, it is possible to retrieve the data by intersecting a reference beam 122 with medium 114 at the same location and at the same angle, wavelength, or phase at which a reference beam 122 was directed during storage of the data. The reconstructed data passes through one or more lenses 134 and is detected by sensor 116. Sensor 116, is for example, a charged coupled device or an active pixel sensor. Sensor 116 typically is attached to a unit that decodes the data.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (“photopolymers”) or a freestanding photorefractive crystal such as iron-doped LiNbO3 crystal.
In the typical photosensitive polymer type holographic storage media, the interference pattern is formed within the media by an irreversible polymerization reaction. In this typical storage media, the matrix does not react during the recording of the hologram to the media. The write components, which are defined as components that react during hologram formation to form the hologram, are separate from the matrix components, which form the matrix. The write components within the matrix, which can include one or more photoreactive monomers, react when exposed to an interference pattern to form a polymer in the exposed regions. The hologram is recorded within the matrix as an index modulation formed between the polymerized write components and the matrix.
The write components can be entirely different chemical compounds than the matrix components. For example, the write components could be chosen so that they react under different conditions than the matrix components. In this way, little reaction of the write components during matrix formation occurs.
Alternatively, the same chemical component can be used as both a matrix component and as a write component. For example, acrylate monomers can be used both as a matrix component, for matrix formation, and as write component, for recording holograms, see for example, U.S. Pat. No. 5,874,187. Although acrylate monomers are used to form both the matrix and to form holograms, the matrix component monomers that react to form the matrix do not substantially react during hologram formation. Although some matrix polymer chains may propagate during hologram writing, the holograph is generally created by new chains that comprise acrylate monomers that are not part of the matrix.
U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, the disclosure of which is hereby incorporated by reference, generally describes several types of photopolymers for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. The matrix components do not substantially react to form a pattern during the recording of the hologram to the media. A monomer, which is not part of the matrix, polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
W. K. Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990, describes a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data. Again, as in U.S. Pat. No. 6,103,454, the host matrix does not substantially react during hologram formation.
The prior art has been concerned with the formation of holograms in a medium in which the matrix is substantially inert during the formation of a pattern in the medium. A medium in which reaction with the matrix is exploited as the method for pattern formation has, until now, not been achieved.
In addition, rewriteable holographic storage media is being developed. For example, U.S. Reissue Pat. 37,658 E, entitled CHIRAL OPTICAL POLYMER BASED INFORMATION STORAGE MATERIAL, describes an optical storage medium in which an optically active polymer is used to store information. The storage medium is optically active at temperatures above Tg and is optically inactive at temperatures below Tg. Information can be repeatedly written to or erased from the optically active polymer by raising the temperature of the optically active medium above Tg. This type of system has the drawbacks of requiring the temperature of the storage medium to be raised above ambient temperatures to store information. Heating of the media can occur by direct absorption of light, however, this can require the use of very high powered lasers and a highly absorptive media.
A storage media that can be used with efficient lasers under ambient conditions and takes advantage of reversible chemical reactions has not yet been achieved.